Generally, with reference to JP-5-85142A, an air conditioning system for a vehicle performs a PID control based on an actual value of an air temperature of a downstream side with respect to an evaporator at an outlet portion of the evaporator of a refrigerant cycle and a target value of the air temperature. In the air conditioning system, the capacity of the compressor is adjustable according to a calculated control value.
However, as shown in FIG. 11, of the present application, in the PID control of the air conditioning system disclosed in JP-5-85142A, when an air conditioner switch becomes ON, a control signal of a predetermined control value G(0) is sent to the compressor so that the compressor is driven at a predetermined output.
The control value G(0) (referring to alternate long and short dashed line in FIG. 11) is the control signal for taking seriously the cooling capability which is necessary in summer or the like. Therefore, with the air conditioner switch being ON, the evaporator temperature will become low (referring to FIG. 11). In this case, when the heat load is a low, the refrigerant flow amount in the refrigerant cycle will become excessively much. Thus, the evaporator temperature will be lower than a target value TEO.
That is, in the case where the temperature in the passenger compartment of the vehicle is high in the summer or the like, the difference between the actual value of the air temperature of the downstream side of the evaporator at the startup of air conditioning and the target value TEO is large, and the capability of the compressor is greatly changed. Therefore, the air temperature of the downstream side of the evaporator will overshoot the target value TEO, with reference to FIG. 11.
In this case, the temperature of the evaporator may become excessively low so that the evaporator freezes. Therefore, bad smell adhered to the evaporator will be liberated to be blown into the passenger compartment along with air-conditioning air, thus discomforting passengers.